1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a transmission/reception apparatus and method using transmit antenna diversity to cope with signal degradation caused by fading.
2. Description of the Related Art
Transmit diversity is one of the many effective technologies designed for relieving fading in a wireless communication system. Typical transmit diversity technology detects desired data symbols using a channel characteristic from a transmitter to a receiver. However, because of the mobility and variation of a channel, it is impossible to correctly detect a channel characteristic between a transmitter and a receiver. Further, feeding back channel state information to the transmitter undesirably results in a reduction in the channel capacity. Therefore, a large amount of research has been conducted on transmit diversity for the case where a transmitter has no channel information.
Recently, space-time block coding (STBC) has attracted a great deal of public attention because it provides good performance when a high data rate is required. In particular, Tarokh et al. proposes a space-block trellis code capable of obtaining both good coding gains and diversity gains when a plurality of antennas are used (see Vahid Tarokh, Et al. “Space time block coding from orthogonal design,” IEEE Trans. on Info. Theory, Vol. 45, pp. 1456-1467, July 1999). Here, the diversity gain corresponds to a reduction in channel gain generated by a fading channel.
FIG. 1 is a block diagram illustrating a conventional transmitter using a space-time block code (STBC). As illustrated in FIG. 1, the transmitter includes a serial-to-parallel (S/P) converter 10, an encoder 20, and N transmission antennas 30-1, 30-2, . . . , 30-N.
Referring to FIG. 1, the S/P converter 10 creates a block of symbols by grouping symbols received from a predetermined information source (not shown) by the N symbols, and provides the created symbol block to the encoder 20. The encoder 20 creates a predetermined number of combinations with the N symbols, and delivers the combinations via the N transmission antennas 30-1, 30-2, . . . , 30-N for their corresponding time periods. The time periods indicate symbol durations.
When 4 transmission antennas are used, symbols output from the encoder 20 can be expressed by a 4*4 encoding matrix shown in Equation (1) below.
                              g          44                =                  (                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                      -                                      s                    2                                                                                                s                  1                                                                              -                                      s                    4                                                                                                s                  3                                                                                                      -                                      s                    3                                                                                                s                  4                                                                              s                  1                                                                              -                                      s                    2                                                                                                                        -                                      s                    4                                                                                                -                                      s                    3                                                                                                s                  2                                                                              s                  1                                                              )                                    (        1        )            
In Equation (1), s1, s2, s3, and s4 are data symbols to be transmitted. Symbols in each column are transmitted for respective time periods, and symbols in each row are transmitted via the respective antennas. Because symbols of one block are transmitted for 4 time periods, the 4 time periods are referred to as “one block duration.” Respective columns of the encoding matrix are orthogonal with one another, and this simplifies coding, helps to decoding and obtains maximum diversity gain.
FIG. 2 is a block diagram illustrating a conventional receiver for receiving the signal transmitted from the transmitter illustrated in FIG. 1. As illustrated in FIG. 2, the receiver includes M reception antennas 40-1, 40-2, . . . , 40-M, a channel estimator 50, a multi-channel symbol arranger 60, and a detector 70.
Referring to FIG. 2, the channel estimator 50 estimates channel coefficients indicating channel gains from the transmission antennas 30-1 to 30-N to the reception antennas 40-1 to 40-M, and the multi-channel symbol arranger 60 collects symbols received by the reception antennas 40-1 to 40-M and provides the collected symbols to the detector 70. The detector 70 detects desired symbols by maximum likelihood (ML) decoding, using hypothesis symbols calculated by multiplying the received signals by the channel coefficients.
In the receiver, a received signal x is expressed by Equation (2) below.
                              r          i                =                                            ∑              i                        ⁢                                          h                i                            ⁢                              s                                  t                  ,                  i                                                              +                      w            t                                              (        2        )            
In Equation (2), t is a symbol duration index (t=0,1, . . . ), and hi indicates channel gain from an ith transmission antenna to the receiver, and is assumed to be an independent complex Gaussian random variable having a variance of 0.5 per real dimension under flat fading. In addition, st,i indicates a symbol output via an ith transmission antenna for a tth symbol duration, and wt is a noise at a tth symbol duration and has an independent zero mean complex Gaussian characteristic having a variance of 1/SNR (Signal-to-Noise Ratio) per complex dimension.
When symbols used during transmission/reception are PSK (Phase Shift Keying) symbols, the symbols are located in a unit circle of a constellation. This means that the symbols are all identical in amplitude. Then, ML decoding of transmitted symbols is equivalent to finding symbols nearest to a linear combination of a received signal r and a channel gain h among all possible symbols.
To decode STBC, information on channel gains is required. When channel characteristic vary fast, it is very difficult to estimate correct information on channel gain, and when a channel characteristic is incorrectly measured, performance of STBC is considerably deteriorated. In order to enable a receiver to effectively estimate a channel characteristic, a transmitter must transmit a training sequence, and the transmission of a training sequence reduces transmission efficiency.
In order to resolve the above-mentioned problem, differential STBC has been developed for which information on a channel characteristic is not required (see H. Jafarkhani, Vahid Tarokh, “Multiple Transmit antenna differential Detection from generalized orthogonal designs,” IEEE Trans. on Info. Theory, Vol. 47, pp. 2626-2631, September 2001).
FIG. 3 is a block diagram illustrating a conventional transmitter using a differential space-time block code (STBC). As illustrated in FIG. 3, the transmitter includes a serial-to-parallel (S/P) converter 105, multipliers 110-1, . . . , 110-K, an adder 115, a delay 120, an encoder 125, and K transmission antennas 130-1, 130-2, . . . , 130-K.
Referring to FIG. 3, the S/P converter 105 parallel-converts a previously transmitted symbol block Sv delayed by the delay 120, and outputs K previous symbols Sv,1, . . . , Sv,K. The multipliers 110-1 to 110-K multiply the K previous symbols by information symbols Pv+1,1, . . . , Pv+1,K to be actually transmitted, respectively, and the adder 115 adds output symbols of the multipliers 110-1 to 110-K, and delivers the result to the delay 120, thereby enabling the result to be multiplied by the next information symbols. Further, the adder 115 provides the added result to the encoder 125. The encoder 125 creates a predetermined number of combinations with output symbols of the multipliers 110-1 to 110-K, and transmits the combinations via the transmission antennas 130-1, 130-2, . . . , 130-K for the corresponding time periods.
An operation of the transmitter will be described with reference to an example where K=4. Initially, the transmitter transmits a particular symbol block S1=[s1,1˜s1,4] having no information according to the encoding matrix. Thereafter, the transmitter transmits Sv=[Sv,1˜Sv,4] in the same manner according to the encoding matrix. When information symbols Pv+1=(Pv+1,1 . . . Pv+1,4) to be transmitted at a time v+1 are received, a transmission symbol Sv+1 is determined by Equation (3) below.
                              S                      v            +            1                          =                              ∑                          k              =              1                        4                    ⁢                                    P                                                v                  +                  1                                ,                k                                      ⁢                                          V                k                            ⁡                              (                                  S                  v                                )                                                                        (        3        )            
That is, information symbols to be transmitted at a time v+1 are multiplied by respective symbols of a symbol block Vk(Sv) transmitted at a previous time v, and then added before being transmitted. Here, the information symbols are a real number created by BPSK (Binary Phase Shift Keying). For the symbol block Vk(Sv), symbol combinations output from the encoder 125 for 4 symbol durations are given by Equation (4),V1(Sv)=(sv,1,sv,2,sv,3,sv,4)T V2(Sv)=(sv,2,−sv,1,sv,4,−sv,3)T V3(Sv)=(sv,3,−sv,4,−sv,1,sv,2)T V4(Sv)=(sv,4,sv,3,−sv,2,−sv,1)T  (4)where T indicates a transposed matrix.
FIG. 4 is a block diagram illustrating a conventional receiver for receiving a signal transmitted from the transmitter illustrated in FIG. 3. As illustrated in FIG. 4, the receiver includes M reception antennas 150-1, 150-2, . . . , 150-M, K delays 155-1, . . . , 155-K, K multipliers 160-1, . . . , 160-K, a symbol arranger 165, and a detector 170.
Referring to FIG. 4, the delays 155-1 to 155-K delay signals previously received for one block duration, and output the delayed signals to the multipliers 160-1 to 160-K. The symbol arranger 165 provides the multipliers 160-1 to 160-K with signals received at the reception antennas 150-1 to 150-M from the transmission antennas 130-1 to 130-K for one block duration. The multipliers 160-1 to 160-K calculate substitution signals by multiplying the receive signals by the previously received signals, and provide the calculated substitution signals to the detector 170. The detector 170 detects an information sequence with the substitution signals according to a corresponding modulation scheme.
In order to describe an operation of the receiver, if the received signal shown in Equation (1) is extended for plural block durations, thenrv,t=h1sv,t,1+h2sv,t,2+h3sv,t,3+h4sv,t,4+wv,t  (5).
In Equation (5), t is a symbol duration index, and v is a block duration index. In the case of STBC using 4 transmission antennas, one block duration is comprised of 4 symbol durations. In addition, wv,t is a noise at a tth symbol duration in a vth block duration. When 4 transmission antennas and the encoding matrix of Equation (1) are used, Equation (5) can be rewritten asrv,1=h1sv,1+h2sv,2+h3sv,3+h4sv,4+wv,1 rv,2=−h1sv,2+h2sv,1−h3sv,4+h4sv,3+wv,2 rv,3=−h1sv,3+h2sv,4+h3sv,1−h4sv,2+wv,3 rv,4=−hlsv,4−h2sv,3+h3sv,2+h4sv,1+wv,4  (6).
By arranging Equation (6), reception signal combinations are created as follows in Equation (7).Rv1=(rv,1,rv,2,rv,3,rv,4)=(sv,1,sv,2,sv,3sv,4)H⊥+(wv,1,wv,2,wv,3,wv,4)Rv2=(−rv,2,rv,1,rv,4,−rv,3)=(sv,2,−sv,1,sv,4−sv,3)H⊥+(−wv,2,wv,1,wv,4,−wv,3)Rv3=(−rv,3,−rv,4,rv,1,rv,2)=(sv,3,−sv,4,−sv,1,sv,2)H⊥+(−wv,3,−wv,4,wv,1,wv,2)Rv4=(−rv,4,rv,3,−rv,2,rv,1)=(sv,4,sv,3,−sv,2,−sv,1)H⊥+(−wv,4,wv,3,−wv,2,wv,1)  (7)
The reception signal combination Rvi is used to detect an ith information symbol. Here, H⊥ indicates channel characteristics in a matrix form, and is defined by Equation (8) as follows.
                              H          ⊥                =                  [                                                                      h                  1                                                                              h                  2                                                                              h                  3                                                                              h                  4                                                                                                      h                  2                                                                              -                                      h                    1                                                                                                -                                      h                    4                                                                                                h                  3                                                                                                      h                  3                                                                              h                  4                                                                              -                                      h                    1                                                                                                -                                      h                    2                                                                                                                        h                  4                                                                              -                                      h                    3                                                                                                h                  2                                                                              -                                      h                    1                                                                                ]                                    (        8        )            
A product of a reception signal at a time v and a reception signal at a time v+1, i.e., a substitution signal, is determined by Equation (9).
                                                                        R                ⁢                                  {                                                            R                                              v                        +                        1                                            n                                        ⁢                                          R                      v                      nH                                                        }                                            =                              R                ⁢                                  {                                                                                    S                                                  v                          +                          1                                                T                                            ⁢                                              H                        ⊥                                            ⁢                                                                                                    H                            ⊥                            H                                                    ⁡                                                      (                                                                                                                            V                                  n                                                                ⁡                                                                  (                                                                      S                                    v                                                                    )                                                                                            T                                                        )                                                                          H                                                              +                                          W                      n                                                        }                                                                                                        =                                                                    ∑                    i                    4                                    ⁢                                                                                                                                      h                          i                                                                                            2                                        ⁢                                          P                                                                        v                          +                          1                                                ,                        n                                                                                            +                                  R                  ⁢                                      {                                          W                      n                                        }                                                                                                          (        9        )            
In Equation (9), R{·} means real conversion, and (·)H indicates Hermitian transpose. Equation (9) represents a substitution signal for calculating an nth information symbol Pv+1,n.
According to Equation (9), all elements except the information symbols Pv+1,1 to Pv+1,4 are real numbers, and the noise information is already known. On a 4-dimention hyper sphere having each of the information symbols as one axis, if a distance from the origin of the information symbols is identical, a receiver can detect information symbol Pv+1,1 to Pv+1,4 even though a value of h indicating a channel characteristic is unknown.
In the conventional transmission/reception system described above, decoding can be performed even though a channel characteristic is unknown. However, symbols in use must be real symbols having the same size. Therefore, in differential STBC technology, transmission symbols are restricted to BPSK symbols. BPSK symbols carry 1-bit information per second per bandwidth (1 bit/hz/sec). However, it is possible to transmit PSK modulation data by grouping transmission symbols by a predetermined number of symbols. For example, it is possible to group 4 symbols by the 2 symbols and then carry 16PSK modulation data on each symbol group. When 2 symbols transmit 16PSK modulation data in this manner, two 16PSK symbols are transmitted for 4 time periods. As a result, it is possible to carry 2-bit information per second per bandwidth (2 bits/hz/sec).
As is well known, M-ary QAM (Quadrature Amplitude Modulation) is more effective than M-ary PSK in view of a signal-to-noise ratio (SNR). In the conventional differential STBC system, PSK must be used even when information of 2 bits or more is carried. Therefore, it is not possible to secure performance improvement attributable to the use of QAM. For example, when 64PSK is used instead of 64QAM, a decrease in SNR accounts to 9.95 dB, and this has a fatal effect on the wireless communication system.